Radio guidance system for mobile craft using automatic homing correction



Feb. 6, 1951 J. AlCARDl 2,540,413

RADIO GUIDANCE SYSTEM FOR MOBILE CRAFT USING AUTOMATIC HOMING CORRECTION Filed May 8, 1945 5 Sheets-Sheet 1 ATTOR/VEY Feb. 6, 1951 J AICARDI 2,540,413

RADIO GUIDANCE SYSTEM FOR MOBILE CRAFT USING AUTOMATIC HOMING CORRECTION Filed May 8, 1945 5' Sheets- Sheet 2 'A V 1 l1f f l 1 11111 I I. A MVP/v70) (70640 A/CA/PO/ 0 6mm 5. O

Feb. 6, 1951 v J. AICARDI 2,540,413

RADIO GUIDANCE SYSTEM FOR MOBILE CRAFT usmc- AUTOMATIC HOMING CORRECTION- Fil'ed May 8, 1945 sshe'ets-sh'e t 4.

uosa x/ IA/CARO/I y .gwnm & 0M

Feb. 6, 1951 J AICARDI 2,540,413

RADIO GU'IDANCE'SYSTEM FOR MOBILE CRAFT USING AUTOMATIC HOMING CORRECTION Filed May 8, 1945 5 Sheets-Sheet 5 //w A/70 J0 552 /9 AMA/m4 6/ gag/mm D Patented Feb. 6, 1951 mrr1-:o sures :-FATENT OFFICE nAiDIo GUID oE SY l (DRAFT USING AUTO RE OTION 59591 1 Aica d Vers ll s n as 'r ar entier snmna y 9i F am S M, FOR MOBILE APPiicationMayS, 1945, Serial'No. 592,-640

311108 t0berf12; 1:944

The present jmtfintiq i relates {to improvements i m ods a sy t m din h m vin ,b odies such,as aircraft by radio.

.It.is,w ell known that inlolindvlanding systems such as are employed at airports, a landing radio-beacon is used ito radiatcia beam for definin Namin p th axi j un t mn suz a manner that the airplane .i-rad rreq y r ,ce es a bath-s s QQI t eaco a d ie zen w-tadiossigna s, which a easily d tin u hab e eith r b rthe thus enabling the airplane o eratpr-,,tol,locate said sd fie en ign 5. nan wh n t haracter ze formas. ninfiame th u rans w m ttin iwmbl mentar so s a ;su :a .dQ

dasltgngs sfcr a lad o beagon -mth ayaqogblemoqulafipnpf id: illeggenc' l e-si na n; t

nd R and: withthe-sameamplitude,

-.b heh mqsi at n ire ensi a theai planeor-zmqvins .n hefl eacen axi QXUQ d-' i ifying 1216911 1161 1 the e :radie uidance methods; 'il'ldfilaterfito the 1 110} @Of that-airplane KQniWhiGhiSj-de .of thezbeacon .:axis= :he is -;loea,ted,:: but the? do not define the zQOllI-I'Dl ita -belappliediin;orde1:tto.1bring the. plane mn-iozsaid ,axis.

The present invention .hassmore particularly cforsits chieflobject to remedy *saidinconvenience :and ate indicate tat everytmoment the direction which theamovingcbody mustwfollow inl 'order wcorrectly to comelonto the axis :of the radiorbeacon.

' For this purpose; thepresent :invention :relates NET-a me'thod for guidingof movingbodies such as-- airplanes,--in which thereyis used a 'radiobeacon defining abe c axis QXTI i method being characterized in that there are combined,

atthe receiver the ijglloyvingsisfla v lu ,thQ B .obtainedji omqthe .radiQ-beacon and i characterizing the impmentary position, of the e nd -beate .aE h --res11ltin com ined m-uthedeft ofathelradio-beacon, -so as to take--into 12, flimv (Cl-1 23 3x107) 191. Q th ai raft moving .abody, must sful'lew ;,i order correctlyg to comelonto thev axis oil-said .acqortiing. tohone-fvorm .of .embodimentiof i-the invention, the obsenved signal *characteri'zingthe course. :of.;;the moving. body with respect to the direction .;ofv the radio-beacon is: added stow or subtracted irommhenbserved signal characterizine t thespositionhofrthe moving .abody withdrespect to the axis of .the radios-beacon according r-ltowthe .phase of the signal!characterizingthe course .of (the :moving body, said :phase indicating whether the course or heading of the mov- ;body. is directed .towardsthe-right or towards accountcthe direction .of said course and to cause ing as Jtheamovina-bodv comes beacon axis moreor less rapidly.

. would follow a correction whichl greater or smaller accofw nearer -to-=-said The invention covers said method irrespectively of the apparatus used for carrying outflthe -same. However, I provide apparatus permitting a particularly simple and efiicacious carrying out of =the= preceding metho -"-;Ihe method according 'to 5 the invention and apparatus for carrying out the same aref shoiivn by --*way *of example I in which:

*in the 4 appended Y drawings =Figures 1 and 2 are "charts employed-for the :purpose of explaining the operation, of, this vention.

i ur 7 1 33 3 t e w ve orm oftheqscillations received *by cyclically alternately connecting two different combinations of receiv ng aerials or antennae.

iF m i=i l s a t wa e l m Phineas- -cillations received by cyclically alternately nnecting .two different combinations of. receiving a n e-W e h ax s o he r dioi ea qni d fin b h tr smi on. f lcom emen ry si n ssu as Q and das es q i ur 5 .r fisellt the cou se Wh sk; the, cra fitherni otsets alcou .e w l g e er m m nt (makes he same ang W h ,t i el tqmthe craf t th beaconq Figure 1 -.is a"circuitidias amss win a rad apparatus,embodyin ii his invention.

Figure '7 is a l circuit diagram .of an auxiliary circuit for the automatic increase of the eiifect of the variation of the course, and

i e v8 a 9am circu d agram zshew 1W0 zsmbodiments pf :a-lradiq uid nc zsvs l-lAccording sto .zthe "method which 1 forms vithe subject matter of the present invention the position indications given by the radio-beacon system are combined with the indications representing the heading of the airplane with respect to the direction of the radio-beacon.

It is well known that with radio-beacons in which complementary code signals are received together, as for instance dots and dashes, the operator on board of the moving craft obtains a detected current having the frequency ;f of the modulation of the transmitter and the amplitude of which varies at the cadence dots-dashes, the amplitude being greater during transmission of dots for the points on one side of the beacon axis and during the transmission of dashes for points on the other side of the beacon axis. For points on the beacon axis, the amplitude remains constant, which gives, when heard, the sensation of a continuous very long dash or unkeyed signal. For a given point the amplitude difference between the observed dots and the dashes increases with the angle a (Figure 1) which the direction OM from the beacon makes with the .beacon axis OK in a manner which is substantially linear so long as a. is small.

On the other hand, it is well known that when one alternately uses on board of a moving body (as an airplane, for instance) for receiving radio signals, either two directional antennae or two combinations of antennae having different directivity patterns, both antennae (or both combinations) having identical properties in the direction of the axis of the airplane, the current supplied by the receiver varies at the rate of the alternation of connection of the two antennae, except when the airplane is moving directly towards the beacon transmitter.

More particularly, it is known that such a device can be obtained by using the combination of a non-directional antenna with a directional coil aerial perpendicular to the axis of the airplane and by periodically reversing the direction in which said combination is made, for instance by reversing the phase of the electromotive force of the coil aerial, so that the electromotive forces induced in the coil aerial and in the non-directional antenna are alternately in phase and in opposition.

Under these conditions, the current delivered by the receiver varies in amplitude at rate of the reversal of the connections of the coil aerial, said variation of amplitude increasing in proportion as the angle B (Figures 2 and 5), which the heading of the airplane at M makes with the direction MO of the beacon transmitter and having opposite directions for two headings MM and MM directed on respective sides of the line MO, increases.

- According to the present invention, the reception of the radio-beacon signal is effected with a device of the kind which has been above referred to and the variations of amplitude of received current due to the effect of the variation 1 of the course of the moving body are combined with the variations of received current due to the position of the airplane with respect to the axis of the radio-beacon, so that the combined course indicator reads zero not when the airplaneis on the beacon axis, but when at any point it is taking the suitable course for correctly getting onto said beacon axis.

Under these conditions, the position of the pointer of the visual indicator changes as soon as the heading or'be-arin'g of the airplane is modi- 'fied;'the' deviation of the pointer (for thelairplane at M) (Figure 2) diminishing in proportion as the axis of the airplane inclines towards the left and becoming zero for a given inclination which is a function of the position of the airplane at M (or, more exactly a function, of the angle MOX) after which it changes its direction if said inclination is exceeded.

For getting onto the beacon axis it is then suflicient for the pilot to handle the plane so as to render and maintain at zero the deviation of the instrument pointer. In proportion as the plane M comes nearer to the beacon axis OX the instrument deviation due to the signal of the radiobeacon diminishes. For maintaining at zero the deviation of the pointer it is necessary to diminish the effect due to the orientation of the plane with reference to the direction of the beacon transmitter. This results in a diminution of the deviation of this orientation, and the course of the plane is brought to coincide with the direction of the axis of the radio-beacon.

Of course, the rapidity with'which the plane gets on the beacon axis OX depends on the manher in which the component efiects are combined and measured.

The phase reversal of the connections of the coil aerial can be obtained by means of an electronic change-over switch the reversing frequency f of which is much smaller than the modulation frequency f of the waves transmitted by the radio-beacon.

With this device, the detection of the received oscillations causes signals to appear having the modulation frequency f and having an amplitude varying at the keying of the dots and dashes; the amplitude of the dots is greater or smaller than that of the dashes according to the side of the beacon axis where the airplane is located. Furthermore, the amplitudes of said signals are modulated at the reversing frequency f thru the phase reversing device of the coil aerial. The extent of this variation is the greater in amplitude the more the heading of the airplane diverges from the direction MO of the beacon.

Thus, as shown in Fig. 4, the amplitude of the signal impulses of the modulation frequency 1 during the reception of a dash I00 will be for example C for the zero phase connection of the coil aerial, and D for the phase connection of the coil aerial. The alternately effective values C and D are produced at the reversing frequency f, during the' entire duration of the dash I00. The successive repetition of the amplitudes C and D of the oscillations of modulation frequency f is effected at this repetition reversing frequency of 1" cycles per second.

The reception of dot [0| shown in Fig. 4 which immediately follows that Of dash I00, first gives for the 180 phase of the connection of the coil aerial, an amplitude E which is smaller than amplitude D if the airplane is on the dash-side of the axis. For the zero phase of the connection of the coil aerial a signal amplitude G is obtained and this amplitude is with respect to amplitude C, for the dash, like E amplitude is with respect to amplitude D.

The so received signals are detected in both of the following independent manners:

(a) Said signals are rectified in order directly to obtain a continuous voltage which varies at the keying rate of the dots and dashes, which voltage is caused to act on a visual indicator, the value and the direction of the variation depending on the position of the plane with respect to the beacon axis.

ache-9,4113

;:.(;b)2;5aid:signais :aredetected andione obtains an. alternating currentuhavingithereversing'zfreuuency f'ysai'd "current being .the greater =ffor a given position of the:airplane:the morethehead- 'ingi diverges", fromifithe; direction of the beacon transmitternbut Zalsoxtvarying itself-.inxamplitude at the .keyingzrate of the dots :anddashes.

By means of a change-over switch, preferably employing electron tubes; synchronized withzthe 'chan'ge-over switchi for the receiving "coil; aerial the: phase of -:said detected pcurrentv of reversin :.frequency f rm'ay be' determined. ;Said phase characterizes sat; a given: "moment .the side ;on

which the beacon transmitter lies-withrespect 1 tothe heading 10f. the airplane. .T Indeed, 'the our- .rent of reversing. frequency f. .becnrnesszero when the airplane .is tdirected'exaetly. acco'rdingto the line MO and takes values which are equalixbut the phases of which difierfrom 1r when the :p'lane takes one of "two.v headings .MM iandflVflVla sym- "metrical with respect toc the :line M: shown .in "HFigure'Z.

Thus, there are involved: a

(a) A code signal direct'current :the intensity "of whichvaries atthe keying"rateuof thedots 1 and dashes while the relative value of the dot and clash for the "periodical variationl'depends --on theposition oftheairplane with :respect to the axis of the radio-beacon;

(b) A currentof a' reversing frequency the amplitude of "which :depends on 'the divergence of the heading of the airplane from the momentary directionxof the radio beacon of the airplane and the phaseofwhich depends on the sense -'of saiddivergence. "The amplitude furthermore undergoes at the keying rate of the dots and'd-ashes periodical variations which are proportional to that of t-he current mentioned "position of the airplane with-respect to'the radiobeacon and the secondrectified-current character'izes the heading of the airplane and thesecurrents are caused simultaneously-1:0 :act upon the indicating apparatus. I

"If the airplane M "is 'on the headingsMM -shown in Figure '2 it is'necessary in-order tocause "the airplane torejo'in the true route to correct 'the heading of the airplane by an angle which *is much greater than if the airplane- 'is on the heading MM. In order -to takethis fact into "account" the second current (1)) is added to "or "subtracted from *thefirst-one (a) according to the phase or sense of the second current b) as determined by whether the 'course MM :orLMM -"of"the'=pla-ne is being-followed.

When the airplane'is-on theheading MM both currents are added; if the airplane follows the "course MM the "two currents are subtracted,

since'there is a phase reversal of the second current.

" "When there is employed a radio beacon with a "double modulation of frequency f and i there are obtained at the output of "the receiver-two currents of frequencies f and F, the amplitude "of the current f predominating on the one side of the beacon axis while the amplitude of-the acurrent f predominates on the other side.

" *If the same coil antenna "device with a periodical phase-reversaho'f the coil aerial at the irelinencykf xis risedascinithe previous. case 'icrkthe reception; aof 'zsaidrsignals, currents; of ireinieisicy f and i will furthermore be modulated withsthe reversingzfrequency f.

. Thesetwo currents f xa-ndj arefthen separated andzeacheofithem .is detected in both; above indicated manners, the first detect-ion giving for each of; them a: direct current which is aiunction :of the position of the airplane with respect togthe axis; Theesecon'ddetectionx ives ioreach ofthe 1. icurrents f and Fan: alternating .currenthaving the reversing frequency f:ofthe; phaseiofqthe connection :iof. :the coil: aerial; .rthezamplitudevof said current varying; with: :the heading ;cf.,,the

- plane.

5, Theixiirect: current obtainedzfrom the-current of ftheirequency-j sis greater .orsmaller than the I rlirect current obtained iromithe wcurrentprhavin the .frequency *7 :acccrdingi to the; sidenaof: t-he beacon axis where the airplane located. .-'-.Uh ere- .iorentheindicator apparatus *such as a: differential :galvanometer (in which both said currents would; 'act. alone would indicate, than :the directionofildefiectionsthe side ofrthe :beacon axis where-the aircraft-Iris :at theemoment.

1 The .ralternating; icurrents having the reversing frequency 7 1 are 'the same- .relation; .ascboth 5 direct .zcurrents; but. their phase varies- 180 according asfthe beacon :transmitter ion the right or on theleft 10f thejaxis {of-the airplane.

, .AUCQI'dil'lgiiOnthB phase of :the, alternating currents having the reversing frequency f thearectifiedcurrent: obtained .from the; difference gbetween the alternatingmurrents.ofa reversingtre- 'quency ii is added topor' subtraotedzf-rom the difference between the two direct currents; act- .ing on the idifierential galmanometer. ,Ehepointer must make it possibleifor the pilotoflthe craft to choose a :suitablepourse for.v correctly .getting backonzthe zbea-conaxi-s.

Inzeither of these..ra:dioguidance;amethodsv with combined signals or with a double modulation frequency, the: 'efiect ;charaicterizing, the .headingnf the airplane is :for. a :ginen {position 10f :the :plane thealgreater :the :more. the route. Ofg' the .planeais inclined to the,.:di-rection- .xifgthe :trans- :mitter {from :the ,plane. 'JgBut this effect, also depends; for a .given orientation offthexheadingon .-the position cfthejplane. :lndeed in'the radio- .beacon :using noteda'sh signals "the .rectifiedacurrent obtained-from.therurrenthavingethereverscing frequency: :f "undergoes; aa= :variation. at. :the

keying iratev of athe 'dot :and dash proportional the variation,;at, this rate;. .of. :the; currenthav- :ing the =moidulating, ;freduency if, that-118130 :say :in the neighborhood of the beacon. axis prop rtional to the angular divergence between'fithe :plane and-the axis of the radio-beacon.

.In1ajradioebeaconisystemwitha double-modulation of the beacon signals, themlifierenwybetweenrtheutwo :rectified currents iobtainedjfrom the currents, of :aureversing.:frequency j f';...coming from the two modulations'f and fi is proportional to the tdifierenceuhetween the :ztwo .xcurrents respectively having?therfrequencyjlgand Hand, accordingly, proportional; :in the neighborhood of the beacon axis, to the angular divergence of the heading :or' the airplane kfromithe axis of the radio-beacon.

' Specifically,- in both--'cases,- in the "neighborhood of the beacon axisg-that is to -say for comparatively small angular itlivergences between the airplaneposition and the beacon axis, *the total energy applied tether-indicating apparatus ire- '75'su1ts' irom:

' '(a) The radio-beacon effect which'is propor- "tional to the angular divergence MOX (Fig. 2),

and

(b) The homing effect which is proportional to effect (a) just mentioned for a given inclination of the route of the airplane to the direction of the beacon transmitter and which, on the other hand, varies in the same sense as said inclina tion.

Thus, the total effect is proportional to the angular divergence MOX (Fig. 2) so long as the inclination of the heading of the airplane tothc line OM between the plane and the beacon transmitter keeps the same value.

Under the operating conditions just described, the heading to be followed by the airplane is defined by the zero value of the resultant combined observed value. With the arrangement just de scribed, if said heading makes at M (Figure 5) the angle B with the line OM it must make the same angle everywhere, that is to say that the airplane thus describes a logarithmic spiral intersecting beacon axis OX according to the angle B. A logarithmic or equiangular spiral is the locus of a point moving along a curve with reference to a pole in such manner that at every point on the curve the tangent makes the same angle with the radius. Now, it is necessary that as the airplane comes nearer to the beacon axis OX, the angle B should diminish and be practically zero when the airplane comes on to the beacon axis OX.

For varying the angle B the voltages due to the angle B, i. e. the homing effect, are caused to vary automatically in proportion as the airplane comes nearer to the axis OX of the radiobeacon, that is to say in proportion as the radio-beacon effect diminishes.

According to another form of employing the method according to the present invention, the homing effect no longer depends on the angular position (1.

Owing to this fact, the homing effect remains fixed for a given bearing of the airplane irrespectively of the angular divergence of the plane with respect to the beacon axis. Thus, the radio-beacon effect, which is a function of the angular position 0., becomes in the neighborhood of the beacon axis OX very small with respect to the homing effect. This results in that the course traversed may have the beacon axis for an asymptote, i. e., it is too long, as a practical matter, in coinciding with said axis.

In order to remedy this inconvenience, the amplification of the homing effect is reduced when the airplane comes nearer to the beacon axis.

Figure 6 shows a radioguidance system which is particularly suitable for carrying out the preceding methods.

This system works with a radio-beacon trans- 'mitting the complementary code dot and dash signals, as already explained.

It comprises, besides the radio-beacon O, the principal groups of units which are indicated below and which are mounted on the airplane M to be guided:

aerial such as a coil antenna l2 turned so as to be perpendicular to the axis of the airplane.

The oscillations introduced into the coil aerial i2 are transmitted through the medium of an electronic change-over switch 3| employing two tricdes l3 and M, to the grid of a mixing tube l5 by means of the high-frequency transformer 32. The anodes of the two tricdes I3 and Hi which form the change-over switch are supplied by an alternating plate voltage having the reversing frequency f. Thus, said triodes work alternately in turns and, accordingly, the phase of the oscillations is periodically reversed in. the transformer 32.

On the other hand, the control grid of the tube I5 receives thru the high-frequency transformer 33 the oscillations induced in the non-directional antenna ll.

Thus, to the control grid of the mixing tube 15 there is applied, at the reversing frequency f, a voltage which is alternately equal to the sum and to the difference of the electromotive forces developed in the coil aerial i2 and in the nondirectional antenna ll.

Said high frequency voltage is amplified after a change of frequency by heterodyne action in the circuits of amplifier tubes l6 and IT. The voltage developed at the output of said amplification stages it and i! is applied by a transformer 34 to the plates of a detector diode I8. The voltage so detected with the modulation frequency f is amplified in the tube l9. Said voltage is then again detected i the two diodes 2B and 2| simultaneously.

The anodes resistance 28 of the diode 2| is shunted by a capacitor 29 of proper value for short-circuiting the reversing frequency f.

Owing to this fact there is directly collected at the ends of the resistance 28 a direct voltage variable at the keying rate of the dot and dash code.

This voltage characterizes the position of the airplane M with respect to the radio-beacon.

This voltage is then amplified by the tube 22 the plate circuit of which supplies the primary winding 37 of an output transformer 21 coupled with the indicating apparatus 38.

The cathode of the diode 20 is connected with the primary winding 39 of a transformer 48 tuned to the reversing frequency f and the secondary winding M of which is connected with tne control grid of an amplifier tube 24. Thus said grid of tube 24 receives the voltage with the reversing frequency f. Said voltage thus amplified by the tube 24 is transmitted by a suitable transformer to the two grids of the tubes 25 and 25 connected in parallel. The tubes 25 and 25 form an electronic change-over switch 43 working in synchronism at reversing frequency f with the change-over switch 3! by virtue of the connection of their anodes to a source of current supply of frequency f.

- According to the phase of the voltage of frequency j which is transmitted to the anode of the tubes 25 and 26 one or the other of these two tubes 25 or 26 conducts current. The tube the anode voltage of which is positive at the same time as the grid voltage, for a suitably adjusted polarization voltage, will, of course, conduct current.

The output current delivered by the tubes 25 and 26 and interrupted at the reversing frequency f, is supplied to the primary windings 4G and d2, respectively, of a transformer 21. The current for these windings is supplied either by thetube 25 or thetube Z'6, .according-to the phase? of the currentof. reversingfrequency f and also according to the bearing'of theairplane withrespect to the direction M0101 the-radio-beacon- The resultant magnetic field: in the secondary winding 45 is according to the case the sumor two impulses ofiopposite directions and very near to each other whichare caused by thepassage from the dash-reception to the dot-reception and then to-the dashereception again;

Thecoils '42 and Mbf-the primary winding of the outputatransforrner .21: to which are delivered .theoutputs of-=the-: two-tubes 25' and 26, respectively, of the electronic change-over switch 32.; are connected in suchza mannerthat their magnetic field act in opposite "directionsonthe secondary" winding 45 iii-r combination with the primary winding-31.

Owing to this fact, the direction of the impulses of. the pointer. of. the "galvanometer St-indicates the direction into which the airplane should. be re-di-rected for proper navigation.

This galvanometer must have a sufiicient in ertiain order that it. isapractically non-responsive to the impulsesofa reversingfrequency f com.-.

ing from the interruptions. of the. anode current supply of both tubes 25'and 26 or from the volt ages of frequencyj which could exist across the terminals of the plate resistance of the diode 2|.

The coefiicients of amplification of the amplifier tubes 22 and 24 can bevaried in order to modify the relative action of the signals defining the angle Band the-angular position a. Thus the rapidity with which the route followed by the airplane will rejoin the axisof the radio-beacon, when the impulses of the visual indicator 38-are maintained at a Zero rate", is reduced.-' For this purpose, pentodes' 22 'and24 having an adjustable coefiicient of amplification can be used;

The relativevalue' of the homing effect can" advantageously be increased automatically when the radio-beacon effectdiminishes. For this purposefbias is applied to' the control grid of the tube 24 as shown the circuit diagram of' Fig-.

ure' 7 'which'shows a'modification applied-to the systemof Figure 6.

In Fig. '7,' the diode '2 l ofthepair of detectors" hasin its plate circuit a resistance-capacity chcuit'35, 36, the time constant of' wh-ich:is'much.i.

longer than the period of the dot-dashkeying of I the signals of the radio beac'on. The negative; voltage at the point A diminishes when the mean:

amplitude of the signals diminishes, that is to say when the airplane comes-nearer to the beacon axis. Accordingly, the grid of the tube 24 is.

polarized less negativelyi This results in an :increase of the amplification. The horning effect is therefore increased.

Figure 8 shows another system for the carryingjout of themethods according to the present invention; in which-theradio-beacon which is used is a radic beacon with a double modulation 10 ofi' 'frequency f and ,f 'of its respective sectors, working as already indicated.

It isxzknownzthat in a radio-beacon with double modulation, each side of the beacon aXisis characterized" by' the predominance of one of the two frequencies of. modulation f and f while the tones of thesefrequencies f and f have the same amplitude whenthe ai plane is substantially on the beacon axis.

tronic change-over switch M- which first adds the coil aerial voltage and the non-directional antenna voltageand' then subtracts the coil' aerial voltage from the non-directional antenna 1 voltage, thisibeing effected ata reversing 'fre-w' quency f.

The thus amplifiedvoltage is detected by diode 52; Two separate circuits tuned respec tively to the frequencies of :modulation f and f are connected one in theanodecircuit and the other in the cathode circuit of the diode 52.

The so detected voltages f and f are'ar'nplified' by the stages ofamplification 53 and 54 and;

respec'-' detected by the doubledi'odes 55 "and-56 tively;

Each double diode 55 and-56 delivers, on the one hand; a direct voltage and, on the other handfi-w a voltage varying with the reversing frequency 7" that is-to be amplified by the amplifier stage 58'.

The direct voltage delivered by each diode 55 and 5B is applied to the winding 66 and to the winding 6! respectively of the multi-winding measuring instrument 51'. Thevoltage with the reversing frequency famplified'by the amplifien stage 58 is appliedto the grids of'the two tubes 59 and H1 of an electronic change-over switch II. This electronic change-over switch 1! works in synchronisrn with the electronic change-over switch 3| comprising the tubes I3 and M which periodically reverse the phase of the 'coil aerial currents in the input circuit of the miXer 5 I. The cathode circuits of the tubes 59 and is of the electronic change-over switch H are connected to the windings 62 and 63, respectively, of the measuring instrument 51.

Owing to this fact, rectified alternating voltage of frequency fof the amplifying stage 58 is supplied either to the winding 62' in the cathode circuit'of the tube 59 or to the winding 63 in the cathode circuit of the tube 1i) according to the phase of said voltage, that is" to say according to the direction of the angular divergence between direction of the heading of the plane and the the radio-beacon from'the plane.

This rectified current acts in combination with T the direct currents'flowing thru the coils 6H and BI upon the pointer of the measuring instrument 51; which is a galvanometer, in one direction or in the other according to the direction of this angular divergence between the heading of the plane and the direction of the beacon.

In this system the rectified current flowing thru the windings E2 or 63 0f the galvanometer 51 is proportional to the difference of amplitude of the currents of frequencies f and f and, accordingly, is proportional, when the plane is in the neighborhood of the beaconing axis, to the angular divergence between the position of the airplane and the axis'of the radio-beacon.

As the heading to be followed'by the pilot is defined by the-zero position of the pointer of the g'alvanometer 51, with the arrangement just described, if said heading makes an angle B with the line M joining the airplane with the radiobeacon, it will make the same angle everywhere and the airplane will describe a logarithmic spiral as shown in Fig. 5.

As a further improvement, the amplification of the tube of the amplifier stage 58, can be varied in order to diminish the angle B in proportion as the plane comes nearer to the beacon axis OX. The bias of said tube 58 can vary, for instance, in the same manner as the mean output voltage delivered from tapped resistor 58a by the double diodes 55 and 56. Thus a higher plate voltage delivered by diodes 55 and 56, corresponding to a greater angular divergence or also corresponds a higher negative bias voltage on the grid of amplifier tube 58, that is to say a reduced amplification of the currents of the reversing frequency f. tive increase of the currents of the reversing frequency f in proportion as the airplane comes nearer to the beacon axis.

In order to thus diminish the angle B, alternatively, one can also proceed in the following manner for a beacon with double modulation f and f such as is employed with the apparatus connected as shown in Figure 9:

The electronic change-over switch 3| comprising the two'tubes l3 and It reverses the phase of the electromotive force of the coil aerial [2 at the reversing frequency f. Thus the input voltage of the amplifier 5| is alternately, as before, the sum and the difference of the electromotive forces in the coil aerial and in the non-directional antenna. At the output of the amplifier 5! a detector diode 66 delivers a voltage of frequency f amplified by the pentode 61. The detector diode B8 is connected to the tuned circuits 8!! and 8| that are tuned to the two frequencies of modulation f and 1 respectively. Voltages of the same frequencies appear in the circuits of the diode 68 and the relative values of these voltages vary according to the position of the airplane with respect to the axis of the radio-beacon.

The diodes 82 and. 83 detect these alternating voltages after said voltages are amplified and filtered in the stages 9| and 90 respectively. The cathode circuits of said diodes 82 and 83 have a time constant which is greater than the reciprocal of the reversing frequency i so that the voltages collected on their terminals are practically direct voltages. Said voltages are applied to two differential windings of the indicator galvanometer 51.

In this arrangement of Fig. 9, the voltage of reversing frequency f amplified by the pentode 61 is transmitted, on the other hand, to the grids of the tubes 85 and 85 which form an electronic change-over switch H driven synchronously with the-change-over switch 3!. The rectified current representing frequency f delivered from tubes 85 and 86 appears according to its phase either on the cathode of the tube 85 or on the cathode of the tube 86 and thus acts upon respective windings of indicator 8'! and upon the pointer of "this indicator 8'! in one direction or in the other according to its phase, that is to say according to the side of the airplane where the beacon transmitter 0- lies.

In proportion as the airplane comes nearer to the beacon axis, the radio-beacon effect dimin- One thus obtains a relaishes and it is necessary to modify the bea ringf of the airplane in order that the effect due 'to-- the reception of the reversing frequency shall remain equal to said radio-beacon effect, that is to say one must progressively bring the heading sition of the airplane with respect to the beacon Owing to this fact, the radio-beacon ef-" axis. fect becomes very small with respect to the effect due to the reception of the reversing frequency f in the neighborhood of the beacon axis. It follows therefrom that the course may have the beacon axis as an asymptote.

In order to remedy this inconvenience, one reduces the amplification of the measured quantity representing the angle B when the airplane comes nearer to the beacon axis.

For this purpose, one controls the amplification of the pentode 61 by its grid bias thru the mean cathode voltage of the diodes 82 and 83. Thus, the amplification increases at the same time as this mean voltage delivered from tapped resistor 51a (Fig. 9).

It diminishes, therefore, when said cathode voltage decreases, this is to say when the craft comes nearer to the beacon axis. It will be apparent that I have provided a novel and very useful system of radio guidance which constitutes a marked improvement over systems heretofore available.

To those skilled in the art it will be evident that the system of my invention is susceptible of modifications of certain features without departing from the spirit of my invention, and all such modifications which are comprehended within the scope of the appended claims I consider to be a part of my invention.

I claim:

1. In radio signal responsive course indicating apparatus for use on board mobile craft in receiving signals from a radio beacon having a beacon axis and directively transmitting two radio-frequency signals carrying respectively two different characteristic signal modulations in two differently directed overlapping lobes on respective sides of said beacon axis, a first receiving antenna having a substantially non-directional characteristic, a second directional antenna fixedly mounted on said craft, radio signal receiving means, cyclically operating signal input means comprising a source of reversing frequency .and means for cyclically combining alternately in opposed polarity at a determined reversing cyclic frequency the signals received on said first and second antennas and to apply their resultant so combined to the input of said receiving means, means for detecting the output of said receiving means and for separatelyderiving therefrom a current varying at said reversing cyclic frequency and another current carrying said characteristic signal modulations, a multiple winding current responsive measuring instrument, and a cyclically operating device connected to apply said current varying at said reversing cyclic frequency to one of the windings of said multiple winding current responsive measuring instrument, means for operating said cyclically operating device in synchronismwith said cyclically operating signal input means, and connections for applying the current carrying ecante said" characteristic signa-l' modulations-to another winding "of said "current responsive -measuring instrument.

2'.'Apparatus asrecited-in *cla imd, said two modulations so carried by said two signals transmitted by said bea'conbeing' respectively dash and dot code signals ofcyclically' repeated determined frequency."

3. Apparatus as recited in-claim l, saidtwo modulations so carried byjsaid two signals transmitted 'to said beacon beingrespectively dash and dot code signals of cyclically repeated determined frequency, and a filter unit interp'ose'd in the output circuitof s'aid means for detecting and deriving said current carrying said dot and dash modulations, and an amplifier interposed in the connection "to' said instrument winding from said means for detecting and deriving said current varying at said reversing cyclic ire-- quency, and a connection from one side ofsaid filter unit to a grid of' saidamplifier-for decreasing the' negative *bias thereof as said derived current carrying said dot anddash modulation decreases as the" craft approaches the beacon 4'. In radio signal responsivecourse' indicating reversing cyclic frequency the signals received orrsaid first 'andseco-ndantennas and to apply their-resultant so combined to the input ofsaid receiving 1neans,-mea=ns for detecting theoutp ut of said receiving-means and for separately eeriving therefrom two-separate currents respectively carryingsi'gnals: of: said: two different identifying modulating frequencies and also for deriving therefrom a th'ir'd"current varying at said reversing cyclic frequency; a multiple Winding current responsive measuring instrument, connections for applying in opposition to respective windings of said instrument said two derived currents respectively carrying signals of said two different identifying modulation frequencies, and connections for applying to other windings of said instrument said third derived current varying at said reversing cyclic frequency.

5. Apparatus as recited in claim 4, and phase responsive cyclic reversing switching means having its frequency positively controlled by said source of reversing frequency and interposed in the path from said detecting and deriving means to said instrument of said current Varying at the reversing frequency for controlling the phase of application of said reversing frequency current to said instrument and causing said instrument to positively indicate the phase of said derived current of reversing frequency with reference to the phase of said source of reversing frequency.

6. Apparatus as recited in claim 4, and voltage divider means having atap for deriving a voltage which is the mean of the output voltages producing said two separate currents respectively carrying signals of said two different identifying modulating frequencies, an amplifier interposed in the connections for applying to other i windings of said instrument said third -de'ri'ved current varying at said "reversing cyclic ire-- quency, and a connection from-'saidf'tap of said voltage divider meansto a grid of said amplifier for increasing the amplification of said amplifier for a small mean value of said two currents'carrying signals of identifying modulations, 7.'Apparatus as recited in claim 4, tecting and deriving means 'being'arranged to first detect and derive said two separate currents respectively carryin-gsignals of said two different identifying modulation frequencies and combining the resultant of the outputs of "said two so identified currents and to subsequently detect and derive from said combined resultant the third current varying at-said: reversing cyclic frequency;

8. Apparatus as set forth in claim 1 further characterized in that the said cyclically operat-' ing signal inputmeans and the said cyclically operating device cornprise electronic discharge devices having their anodes connectedtogether.

9. A systernfor radio-guiding of aircraft 'conrprising a radio beacon having anantenna system and meansfor transmitting signalener-gy to define a beacon axis,- receiving-apparatus carried by the aircraft for receiving signals" transmitted by said" radio beacon,- said receiving apparatus including a directional'antenna and-a substantially non-directional antenna, a mixertube coupled to said substantially non-directional am tenna,- an electronic phase-shifting switch connected between saiddirectional antenna and said mixertube,- means for operating said electronic phase shifting switch at a predetermined frequency so that the signal voltage applied to said'lnixer tube will'b'e alternately, at said predetermined frequency; equal to the sum'and to the difference of the signal electromotive forces developed in said directional antenna and in said substantially non-directional antenna, means for amplifying said signal voltage, a detector for detecting said signal voltage, an electromagnetic device and an indicator controlled thereby, a second electronic phase shifting switch connected between said detector'and said electromagnetic device, and connections for operating said second electronic phase shifting switch in synchron-ism with said first mentioned electronic phaseshiftingswitch in synchronism with said 10. A system for radio guiding of aircraft comprising a radio beacon having an antenna system and means for transmitting signal energy to define a beacon axis, and for transmitting different signals on each side of said beacon axis, receiving apparatus carried by the aircraft for receiving signals transmitted by said radio beacon, said receiving apparatus including a directional antenna and a substantially non-directional antenna, a mixer tube coupled to said substantially non-directional antenna, an electronic phase shifting switch connected between said directional antenna and said mixer tube, means for operating said electronic phase shifting switch at a predetermined frequency so that the signal voltage applied to said mixer tube will be alternately, at said predetermined. frequency, equal to the sum and to the difference of the signal electromotive forces developed in said directional antenna and in said substantially nondirectional antenna, means for amplifying said said de-' signal voltage, a detector for detecting said signal voltage, an electromagnetic device and an indicator controlled thereby, a second electronic phase shifting switch connected between said detector and said electromagnetic device, connections for operating said second electronic phase shifting switch in synchronism with said first mentioned electronic phase shifting switch, and additional connections for supplying the aforesaid different signals from the sides of said radio beacon axis to said electromagnetic device to produce indications in said indicator characteristic of the position of the aircraft with respect to said radio beacon axis.

11. A system for radio guiding of aircraft comprising a radio beacon having an antenna system and means for transmitting signal energy to define a beacon axis, and for transmitting different signals on each side of said beacon axis, receiving apparatus carried by the aircraft for receiving signals transmitted by said radio beacon, said receiving apparatus including a directional antenna and a substantially non-directional antenna, a mixer tube coupled to said substantially non-directional antenna, an electronic phase shifting switch connected between said directional antenna and said mixer tube, means for operating said electronic phase shifting switch at a predetermined frequency so that the signal voltage applied to said mixer tube will be alternately, at said predetermined frequency, equal to the sum and to the difference of the signal electromotive forces developed in said directional antenna and in said substantially nondirectional antenna, means for amplifying said signal voltage, rectifier means for rectifying said signal voltage, an electromagnetic device and an indicator controlled thereby, a filter tuned to said predetermined frequency connected to said rectifier means, a second electronic phase shifting switch connected between said filter and said electromagnetic device, and connections for operating said second electronic phase shifting switch in synchronism with said first mentioned electronic phase shifting switch to produce indications in said indicator characteristic of the position of the aircraft with respect to said radio beacon axis.

12. A system for radio guiding of aircraft comprising a radio beacon having an antenna system and means for transmitting signal energy to define a beacon axis, and for transmitting different modulation signals on each side of said beacon axis, receiving apparatus carried by the aircraft for receiving signals transmitted by said radio beacon, said receiving apparatus including a directional antenna and an omni-directional antenna, a mixer circuit coupled to said substantially non-directional antenna, a phase shifting switch connected between said directional antenna and said mixer circuit, means for operating said phase shifting switch at a predetermined frequency so that the switch voltage applied to said mixer circuit will be alternately, at said predetermined frequency, equal to the sum and to the difference of the'signal electromotive forces developed in said directional antenna and in said omni-directional antenna, means for amplifying said signal voltage, rectifier means for rectifying said signal voltage, an electromagnetic device and an indicator controlled thereby, a filter tuned to said predetermined frequency connected to said rectifier means, a second phase shifting switch connected between said filter and said electromagnetic device, connections for operating said second phase shifting switch in synchronism with said first mentioned phase shifting switch, and means for applying signals corresponding to said modulation signals to said electromagnetic device to produce indications in said indicator characteristic of the position of the aircraft with respect to said radio beacon. I

JOSEPH AICARDI.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,484 Cooke et a1. May 23, 1946 2,286,804 Hooven June 16, 1942 2,327,641 Hooven Aug. 24, 1943 2,393,624 Ferrill, Jr. Jan. 29, 1946 2,395,854 Ferrill, Jr. Mar. 5, 1946 2,423,337 Moseley July 1, 1947 FOREIGN PATENTS Number Country Date 516,567 Great Britain Jan. 5, 1940 

